I recently acquired a K6/2 400MHz system, complete with VIA VA-503+ motherboard, GeForce2 MX200 in AGP, 74GB HD (the BIOS got patched as of last night to read > 32GB drives), and just recently a Realtek 10/100 PCI NIC. My question is, how would I go about overlocking one of these? I have a heatsink installed and I have a case fan cooling the CPU.

Most likely using the jumpers on the motherboard, select FSB/multiplier. I doubt you have many options on it though.
I guess you don't have the manual of the board, try getting it on google or at least it's jumper settings.

Something ive experienced with older systems that you might want to know is.. When you OC too far the computer will just stop turning on sometimes. You have to put the jumpers back to the default positions even after a cmos clearing.